How to do an event on a Button?

Can anyone give me an example how to do another window when click to a button in a window?
Please whitout any other plugin like FormAPI, just an example whit this code:

PHP:

$handler = new Handler();
                $packet = new ModalFormRequestPacket();
                $packet->formId = $handler->getWindowIdFor(Handler::WINDOW);
                $packet->formData = $handler->getWindowJson(Handler::WINDOW, $this, $player, $mandar);
                $player->dataPacket($packet);
                return true;
 

Example:

PHP:

       Menu1                                     Menu2
[click me to go menu2] <player clicks> then  [SendMessage] <player clicks> then $player->sendMessage("hi");


 

i didn't read how those new things called Froms works but i have seen peoples using Packets receiving to get the result.